U.S. patent number 4,946,244 [Application Number 07/322,529] was granted by the patent office on 1990-08-07 for fiber optic distribution system and method of using same.
This patent grant is currently assigned to Pacific Bell. Invention is credited to John J. Schembri.
United States Patent |
4,946,244 |
Schembri |
August 7, 1990 |
**Please see images for:
( Certificate of Correction ) ** |
Fiber optic distribution system and method of using same
Abstract
A method and apparatus are disclosed for providing a fiber optic
distribution network between a central office and a group of users
generally in a localized area. The apparatus utilizes a continuous
optical fiber primary loop in communication with the central office
and passing in the vicinity of each user. Each user is connected to
an optical fiber in the primary loop in a manner which provides
protection against breaks in the primary loop. Embodiments of the
apparatus in which a plurality of users are connected to a single
optical are also disclosed.
Inventors: |
Schembri; John J. (Danville,
CA) |
Assignee: |
Pacific Bell (San Francisco,
CA)
|
Family
ID: |
26710531 |
Appl.
No.: |
07/322,529 |
Filed: |
March 13, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
34087 |
Apr 2, 1987 |
4871225 |
|
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|
Current U.S.
Class: |
398/82; 385/32;
385/50; 398/1; 398/127; 398/153; 398/167.5 |
Current CPC
Class: |
G02B
6/28 (20130101); H04B 10/2755 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); H04B 10/213 (20060101); G02B
006/28 (); G02F 001/00 (); H04B 009/00 () |
Field of
Search: |
;350/96.15,96.16,96.21,96.20,96.22,320,96.23
;370/1,3,4,85,86,87,88,89 ;250/227.11
;455/601,600,602,606,603,607,604,612,617 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Healy; Brian
Attorney, Agent or Firm: McCubbrey, Bartels, Meyer &
Ward
Parent Case Text
The present invention relates to fiber optic communication networks
for use in telephone systems, and more particularly to a
distribution system which provides superior redundant communication
facilities.
This application is a Continuation-In-Part of U.S. Pat. Application
Ser. No. 034,087 filed April 2, 1987, now U.S. Pat. No. 4,871,225,
by John Schembri and entitled "FIBER OPTIC DISTRIBUTION NETWORK".
Claims
What is claimed is:
1. A fiber optic distribution system for providing communication
access between a central office and a plurality of users generally
in a localized area, said fiber optic distribution system
comprising:
an optical fiber primary loop including one or more optical fibers,
said primary loop leaving said central office, passing in the
vicinity of each user of said plurality of users, and returning to
said central office, and
at least one or more patch means arranged in said primary loop for
coupling a selected user with the central office through said
primary loop, each said patch means including means for
interrupting an optical fiber in said primary loop to create first
and second optical fiber segments, each said optical fiber segment
providing a bi-directional communication path between said user and
said central office, each said patch means including first light
transmitting means for transmitting light signals of a first
predetermined wavelength to said central office on said first
optical fiber segment and first light detecting means for receiving
light signals of a second predetermined wavelength from said
central office on said first optical fiber segment, said first and
second wavelengths being chosen such that no two said patch means
coupled to the same optical fiber utilize the same first and second
wavelength.
2. The fiber optic distribution system of claim 1 wherein each said
patch means further comprises second light transmitting means for
transmitting light signals of a third predetermined wavelength to
said central office on said second optical fiber segment and second
light detecting means for receiving light signals of a fourth
predetermined wavelength from said central office on said second
optical fiber segment, said third and fourth wavelengths being
chosen such that no two said patch means coupled to the same
optical fiber utilize the same third and fourth wavelength.
3. The fiber optic distribution system of claim 2 wherein said
first wavelength is the same as said third wavelength.
4. The fiber optic distribution system of claim 3 wherein said
second wavelength is the same as said fourth wavelength.
5. The fiber optic distribution system of claim 2 wherein said
patch means further comprises control mean, said control means
comprising:
means for receiving electrical signals from a said user and for
coupling said received signals to said first and second light
transmitting means;
means for transmitting electrical signals received on a selected
one of said first or second light detecting means to said user;
error detecting means for detecting transmission errors on the said
optical fiber segment on which said selected detector detects light
signals, said error detecting means including means for causing
said selected one of said first and second light detecting means to
be changed to the other of said first and second light detecting
means when a said transmission error is detected.
6. The fiber optic distribution system of claim 5 wherein said
patch means further comprises means for maintaining continuity
between said first and second optical fiber segments.
7. The fiber optic distribution system of claim 1 wherein different
portions of said optical fiber primary loop are substantially
spaced apart to reduce the probability that an event leading to an
interruption in said optical fiber primary loop will result in said
optical fiber primary loop being interrupted in more than one
place.
8. A method for providing communication access between a central
office and a plurality of users generally in a localized area, said
method comprising the steps of:
providing an optical fiber primary loop including one or more
optical fibers, said primary loop leaving said central office,
passing in the vicinity of each user of said plurality of users,
and returning to said central office, and
interrupting an optical fiber in said primary loop by the insertion
of at least one or more patch means therein to create first and
second optical fiber segments, each said optical fiber segment
providing a bi-directional communication path between said user and
said central office, said patch means including first light
transmitting means for transmitting light signals of a first
predetermined wavelength to said central office on said first
optical fiber segment and first light detecting means for receiving
light signals of a second predetermined wavelength from said
central office on said first optical fiber segment, said first and
second wavelengths being chosen such that no two said patch means
coupled to the same optical fiber utilize the same first and second
wavelength.
9. The method of claim 8 wherein said patch means further comprises
second light transmitting means for transmitting light signals of a
third predetermined wavelength to said central office on said
second optical fiber segment and second light detecting means for
receiving light signals of a fourth predetermined wavelength from
said central office on said second optical fiber segment, said
third and fourth wavelengths being chosen such that no two said
patch means coupled to the same optical fiber utilize the same
third and fourth wavelength.
10. The method of claim 9 wherein said first wavelength is the same
as said third wavelength.
11. The method of claim 10 wherein said second wavelength is the
same as said fourth wavelength.
12. The method of claim 9 wherein said patch means further
comprises control means, said control means comprising:
means for receiving electrical signals from a said user and for
coupling said received signals to said first and second light
transmitting means;
means for transmitting electrical signals received on a selected
one of said first or second light detecting means to said user;
error detecting means for detecting transmission errors on the said
optical fiber segment on which said selected detector detects light
signals, said error detecting means including means for causing
said selected one of said first and second light detecting means to
be changed to the other of said first and second light detecting
means when a said transmission error is detected.
13. The method of claim 12 wherein said patch means further
comprises means for maintaining continuity between said first and
second optical fiber segments.
14. The method of claim 8 wherein different portions of said
optical fiber primary loop are substantially spaced apart to reduce
the probability that an event leading to an interruption in said
optical fiber primary loop will result in said optical fiber
primary loop being interrupted in more than one place.
Description
Improvements in fiber optic cables have made such cables an
attractive alternative to metallic conductors for providing
telephone and other communication services that function through a
telephone central office or the like. The individual fibers have
enormously greater bandwidths than metallic conductors, and hence
are capable of providing communication services which were not
previously possible.
In a typical fiber optic based telephone system, cables comprising
bundled optical fibers are laid from a telephone central office
along routes that pass in the vicinity of potential subscribers.
When a new user subscribes to the telephone service, a pair of
fibers in one of the cables is cut in the vicinity of the new
subscriber. The portions of the fibers between the subscriber and
the central office are used to provide a bi-directional
communication path between the subscriber and the central office.
Typically, one fiber is used to send information in a digital
format from the central office to the subscriber's premises, and
the other is used to send digital information from the subscriber's
premises to the central office.
The portions of the fibers between the subscriber and the end of
the cable that is not connected to the telephone central office are
wasted. The amount of wasted optical fiber can be a significant
fraction of the total cable. Consider a cable starting at the
central office which is to service the geographic area from the
central office to some distant point. Further assume that potential
users are uniformly distributed along the route of the cable in
question. In this case, approximately one half of the total fiber
length will be wasted when all of the users are connected.
As the amount of information carried on an individual communication
link increases, the need for security against loss of the
communication link also increases. For example, fiber optics make
it possible to provide high speed data communications services
within the telephone system for use in computer networks. However,
if the fiber cable connecting an important computer in the data
processing system to the other computers therein is cut, the entire
data processing network may be rendered useless. The economic
consequences of such a loss can be severe. Hence, communication
systems which provide redundant communication which protects
against such losses are highly advantageous.
One prior art method for providing redundant communication utilizes
two sets of fibers in the cable between the user and the central
office. In this scheme, two fibers are used to provide
communication from the central office to the subscriber, and two
fibers are used to provide communication from the subscriber to the
central office. This type of system provides security against loss
of communication on an individual fiber; however, it does not
provide protection against loss of the entire cable. Such cable
losses can occur during construction work when excavation equipment
inadvertently breaks a cable.
The cost of providing four individual fibers for each user also
detracts from such schemes. The fiber optic cables represent a
significant portion of the cost of a communication system based on
fiber optics. Hence, it would be advantageous to provide a
distribution network that allowed more than one subscriber to be
connected to each optical fiber. However, as the number of users
serviced by any given fiber increases, the need for security
against cable breaks also increases.
Broadly, it is an object of the present invention to provide an
improved fiber optic communication system.
It is a further object of the present invention to provide a fiber
optic communication system which makes more efficient use of the
fibers in the fiber optic cables.
It is yet another object of the present invention to provide a
fiber optic communication system which provides protection against
a cable interruption. for receiving light signals of a second
predetermined wavelength from the central office on the first
optical fiber segment. The patch box also includes a second light
transmitter for transmitting light signals of a third predetermined
wavelength to the central office on the second optical fiber
segment and a second light detector for receiving light signals of
a fourth predetermined wavelength from the central office on the
second optical fiber segment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a typical prior art fiber optic communication
system for providing telephone and related services between a
central office and a number of system users.
FIG. 2 illustrates a fiber optic distribution system according to
present invention.
FIG. 3 illustrates the preferred manner in which a redundant
service user is connected to two fiber optic segments in a fiber
optic distribution system according to the present invention.
FIG. 4 illustrates a second embodiment of a fiber optic
distribution system according to the present invention which
provides for more than one user on each fiber of the primary fiber
optic cable.
FIG. 5 is a more detailed illustration of the interface unit 420
shown in FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
It is still another object of the present invention to provide a
fiber optic communication system which allows multiple subscribers
to be connected to a single optical fiber while providing
protection against a cable interruption.
These and other objects of the present invention will become
apparent from the following detailed description of the present
invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention comprises a fiber optic distribution system
for providing communication access between a central office and a
plurality of users generally in a localized area. The fiber optic
distribution system includes an optical fiber primary loop having
one or more optical fibers. The primary loop leaves the central
office, passes in the vicinity of each user of said plurality of
users, and returns to the central office. The distribution system
also includes a patch box arranged in the primary loop for coupling
a selected user with the central office through the primary loop.
The patch box includes means for interrupting an optical fiber in
the primary loop to create first and second optical fiber segments.
Each of the optical fiber segments provides a bi-directional
communication path between the user and the central office. The
patch box includes a first light transmitter which transmits light
signals of a first predetermined wavelength to the central office
on the first optical fiber segment and a first light detector.
FIG. 1 illustrates a typical prior art fiber optic communication
system for providing telephone and related services between a
central office 12 and a number of system users of which users 14,
16, and 18 are typical. Users 14 and 16 are connected to central
office 12 by fibers in a fiber optic cable 11. User 18 is connected
to central office 12 by fibers in a second fiber optic cable 13.
Each fiber optic cable comprises a plurality of optical fibers of
which fiber 17 is typical. The fiber optic cables are initially
laid from central office 12 along routes which pass in the vicinity
of potential users. When a new user subscribes to the telephone
service, a pair of fibers in the cable passing near the user in
question are cut. The new user is then connected to the portions of
these fibers which run between the central office and the user in
question. For example, user 14 is connected to the central office
by fiber segments 19. The remaining portions of the fibers in
question are wasted. The wasted fiber segments corresponding to
fiber segments 19 are shown at 15 in FIG. 1.
At each user, the fiber which provides communications from the
central office to the user terminates in a light detector which
converts the light signals on the fiber to electrical signals. The
fiber which is used to send signals from the user to the central
office is connected to a light source which is typically a laser or
a light emitting diode. Similarly the receiving and transmitting
ends of the fibers at the central office terminate with a light
detector and a laser, respectively.
Telephone service may be lost if either one of the fibers is
broken. As was pointed out above, such breaks often occur during
construction work. In addition, service may be lost if either of
the light detectors or the lasers become non-functional.
For those users requiring added security against service
interruptions arising from fiber failures, additional fibers are
assigned. For example, user 18 is connected to two sets of fibers
22 and 24. Such redundant service protects user 18 against a
failure in one of the fibers. Such failures arise when the fiber
splices making the connections between the cut ends of the fibers
in cable 13 and the user 18 are disturbed. In addition, this type
of redundant connection protects the user from interruptions
resulting from laser or light detector failures.
Unfortunately, this type of redundant service does not protect user
18 against a break in the entire cable. If cable 13 were cut at
location 26, both sets of fibers connecting user 18 with central
office 12 would be lost. As noted above, such cuts often occur when
construction equipment is used to excavate areas in the vicinity of
the fiber optic cables.
The present invention provides protection against service
interruptions caused by such cable cuts and reduces the amount of
optical fiber wasted when new subscribers are connected to the
telephone system. A fiber optic distribution system according to
the present invention is illustrated in FIG. 2 at 35. Fiber optic
system 35 connects a number of telecommunication users to a central
office 40. Typical users are shown at 42, 44, and 46. The users are
connected to central office 40 by means of a fiber optic cable 48
which comprises a plurality of optical fibers of which fiber 50 is
typical.
Cable 48 is laid in a loop configuration. That is, it leaves
central office 40, passes in the vicinity of potential subscribers,
and then returns to central office 40. The route taken by cable 48
is preferably chosen to insure that different portions of cable 48
are geographically separated from one another. This geographic
separation insures that the probability of inadvertently cutting
cable 48 in more than one place in a single event is minimized.
Cable 48 serves two types of telephone users, those requiring
redundant service for security against optical fiber failures and
those requiring only normal telephone service. User 42 shown in
FIG. 2 is provided with redundant service, while users 44 and 46 ar
provided with only normal service.
In the preferred embodiment of the present invention, a user
receiving normal service is connected to central office 40 by a
single fiber. The fiber is used to provide a bi-directional
communication path with the central office by utilizing wavelength
multiplexing. That is, data is sent from the user to the central
office using one color of light and data is sent from the central
office to the user using a different color of light. The user is
connected by interrupting one of the fibers in cable 48.
This interruption creates two fiber segments, one connecting the
user to the central office by a first geographic route and one
connecting the user to the central office by a second geographic
route. When only normal service is desired, one of these two
segments is selected for connecting the user to the central office.
The remaining segment is then available for use in connecting a
second user to the central office by the other geographic
route.
For example, user 44 is connected to central office 40 by making a
cut 61 in a fiber to form segment 60 connecting user 44 to the
central office. The remaining portion of the fiber in question is
available for connecting another user to the central office. This
portion is used to connect user 46 to the central office. To
connect user 46 to the central office, a second cut 63 is
introduced in the fiber in the vicinity of user 46. This cut
creates fiber segments 62 and 64. Segment 64 connects user 46 to
central office 40. Segment 62 is wasted.
The amount of wasted fiber can be minimized by careful selection of
the pairs of users connected by any given fibers. It will be
apparent to those skilled in the art that the waste is minimized by
choosing users who are geographically close to one another.
In the present invention, if a user requires redundant service,
e.g., user 42, then the user is connected to central office 40 by
two different geographic paths. When user 42 is connected to the
distribution system, a fiber which has not been interrupted is
chosen. A break 65 is introduced in the fiber in the vicinity of
the user, creating two segments 52 and 54. User 42 is connected to
each segment using the same type of wavelength multiplexing system
described above for non-redundant service users. Segment 52
provides a bi-directional communication path to central office 40
by a first geographic route, and segment 54 provides a
bi-directional communication path to central office 40 by a second
geographic route.
The preferred manner in which a redundant service user is connected
to the two fiber optic segments is illustrated in FIG. 3 at 70.
FIG. 3 shows a single user 72 connected to a central office 74 by
two fiber optic segments 76 and 78 created by cutting a single
fiber loop in the vicinity of user 72.
User 72 is connected to segments 76 and 78 by a patch box 80. Patch
box 80 includes two fiber adapters 81 and 82. Each fiber adapter
provides a means for interfacing light signals to and from the
fiber connected thereto. Fiber adapter 81 interfaces signals to and
from segment 76, and fiber adapter 82 interfaces signals to and
from segment 78.
Each of the segments 76 and 78 provides an independent
bi-directional communication path between user 72 and central
office 74. Signals from user 72 to central office 74 are sent on
segments 76 and 78 using lasers 83 and 84, respectively. Each laser
transmits signals using a first predetermined wavelength. These
signals are detected in central office 74 by detectors 101 and 102
in patch box 89 located in central office 74. The filters 103 and
105 are used to remove light of other wavelengths from the light
exiting each segment. For example, filter 103 guarantees that light
from laser 85 which is reflected back toward detector 101 by the
inner surface of adapter 107 is not detected by detector 101. In
this way, it is assured that detector 101 only detects light
transmitted by laser 83 in patch box 80.
Similarly, signals from central office 74 to user 72 are sent by
lasers 85 and 86 in central office 74 and received by detectors 87
and 88 in patch box 80. These signals are sent at a second
predetermined wavelength which is different from said first
predetermined wavelength. Filters 99 and 100 are used to remove all
light which does not have a wavelength equal to said second
predetermined wavelength.
The outputs of detectors 87 and 88 are inputted to a controller 104
which selects one of the outputs for transmission to user 72 on
line 110. At any given time, controller 104 selects one of the two
segments 76 and 78 for use in transmitting signals to and from
central office 74. If controller 104 detects an interruption in
service on the segment in question or an unacceptable bit error
rate, controller 104 switches to the other segment.
Similarly, controller 104 receives signals on line 111 from user 72
for transmission to central office 74. Controller 104 outputs these
signals to both of lasers 83 and 84. As noted above, these signals
are received by detectors 101 and 102 which output detected signals
to controller 115 in patch box 89 central office 74. Controller 115
operates in a manner analogous to controller 104 in patch box 80.
Controller 115 selects the signal from the currently used segment
for transmission to central office 74 on line 120. Similarly,
central office 74 transmits signals to controller 115 on line 121
for transmission to user 72. Controller 115 couples these signals
to lasers 85 and 86. In the preferred embodiment of the present
invention, the currently active segment selected by controller 115
does not depend on the active segment chosen by controller 104.
The embodiment of the present invention illustrated in FIG. 2
utilizes one fiber loop of cable 48 for connecting each user to
central office 40. A second embodiment of the present invention
which provides for more than one user on each fiber of the cable is
illustrated in FIGS. 4 and 5. FIG. 4 illustrates the manner in
which each of three users 401, 402, and 403 is connected to a
central office 405 by a single optical fiber 406 which comprises
segments 407-410. Each user is connected to fiber 406 by cutting
fiber 406 and inserting an interface unit of which interface unit
420 is typical.
Each user communicates with central office 405 using a unique pair
of wavelengths in a manner analogous to that described above with
reference to FIGS. 2 and 3. That is, signals are sent to the
central office via two geographic routes using light of a first
predetermined wavelength and signals are received from the central
office via two different geographic routes by using light of a
second wavelength. The values of said first and second
predetermined wavelengths are different for each user; hence, the
privacy of the communication between central office 405 and each
user is assured. In addition, the use of different wavelengths
provides each user with the maximum bandwidth of the optical fiber
at any given wavelength.
A more detailed diagrammatic view of interface unit 420 is shown in
FIG. 5. Interface unit 420 is inserted into fiber 406 with the aid
of coupling units 444. Such optical couplers are conventional in
the fiber optic arts. Interface unit 420 maintains the continuity
of fiber 406. That is, light signals generated by other users or
the central office are not blocked by interface unit 420. Interface
unit 420 includes two "Y" shaped light pipes 422 and 424. Light
pipe 422 splits the light traveling in fiber 406 in the direction
of arrow 40 such that a portion of said light is incident on a
detector 432. The remaining light continues down fiber 406. In
addition, light pipe 422 directs light generated by a laser 434
into fiber 406 such that said light traverses fiber 406 in the
direction of arrow 436.
Similarly, light pipe 424 splits the light traversing fiber 406 in
the direction of arrow 440 such that a portion of said light is
incident on a detector 442. In addition, light pipe 424 directs
light generated by a laser 446 down fiber 406 in the direction
indicated by arrow 448.
Detectors 432 and 442 detect light of a first predetermined
wavelength and lasers 434 and 446 generate light of a second
predetermined wavelength. The values of said first and second
wavelengths are different for each user. Hence, any given user can
only receive communications directed to that user.
Interface unit 420 also includes a controller 450 which receives
the outputs of detectors 432 and 442. Controller 450 selects one of
these outputs for transmission to the user on line 451. Controller
450 also receives signals from the user on line 452 and couples
those signals to lasers 434 and 446.
Controller 450 includes circuitry for detecting errors in the data
received by detectors 432 and 442. This error detection circuitry
preferably recognizes bit transmission errors as well complete loss
of signal. Such error detection circuitry is conventional in the
electronic arts. At any given time, controller 450 defines one of
detectors 432 and 442 as the active detector. Since each laser and
detector pair communicate with the central office utilizing a
different portion of fiber 406, the choice of active detector
defines which portion of fiber 406 is currently utilized for
communication with central office 405.
When the bit error rates exceed a predetermined threshold, or when
reception is lost by the currently active detector, controller 450
automatically switches to the other detector. Hence, if a break
occurs in fiber 406 which interrupts transmission on the currently
active communication path, communication is automatically switched
to the other communication path to central office 405 which is
routed via a different portion of fiber 406. Since the different
portions of fiber 406 preferably run along different geographic
routes, a single break in fiber 406 will, at most, interrupt
communication on one of detectors 432 and 442.
Central office 405 includes two detectors and two lasers for each
of the users 401-403. The lasers corresponding to user 401 are
shown at 461 and 462, respectively. The lasers corresponding to
user 402 are shown at 463 and 464, respectively. And, the lasers
corresponding to user 403 are shown at 465 and 466, respectively.
Similarly, the detectors corresponding to user 401 are shown at 471
and 472, respectively. The detectors corresponding to user 402 are
shown at 473 and 474, respectively. And, the detectors
corresponding to user 403 are shown at 475 and 476,
respectively.
Lasers 461, 463, and 465 transmit light down segment 407 of optical
fiber 406 in the direction indicated by arrow 481. The wavelengths
at which these lasers transmit will be denoted by L.sub.1, L.sub.2,
and L.sub.3, respectively. Similarly, lasers 462, 464, and 466
transmit light down segment 410 of optical fiber 406 in the
direction indicated by arrow 482. The wavelengths at which these
lasers transmit are also L.sub.1, L.sub.2, and L.sub.3,
respectively.
The detectors in each of the interface units 420 at the user's
premises are tuned to the corresponding laser wavelength. That is,
the detectors in the interface unit at user 401 detect only light
of wavelength L.sub.1. The detectors in the interface unit at user
402 detect only light of wavelength L.sub.2, and so on. The
wavelengths detected are preferably determined by optical filters
incorporated in each detector. PG,17
Detectors 471, 473, and 475 are coupled to segment 407 of fiber 406
by a light pipe 490 which illuminates each detector. These
detectors receive light traveling in the direction shown by arrow
483. Each detector is equipped with a filter which assures that the
detector in question only detects light of a specified wavelength.
The wavelengths of the light detected by detectors 471, 473, and
475 will be denoted as D.sub.1, D.sub.2, and D.sub.3, respectively.
Similarly, detectors 472, 474, and 476 are coupled to segment 410
of fiber 406 by a light pipe 491 which illuminates each detector.
These detectors receive light traveling in the direction shown by
arrow 484. Each detector is equipped with a filter which assures
that the detector in question only detects light of a specified
wavelength. The wavelengths of the light detected by detectors 472,
474, and 476 are D.sub.1, D.sub.2, and D.sub.3, respectively.
The lasers in each of the interface units 420 are tuned to transmit
the corresponding wavelength. That is, the lasers in the interface
unit 420 at user 401 transmits light of wavelength D.sub.1. The
lasers in the interface unit 420 at user 402 transmit light of
wavelength D.sub.2, and so on.
Detectors 471-476 and lasers 461-466 are coupled to a controller
495 in central office 405. Controller 495 includes one output line
and one input line for each user. The pair of lines for user 401
are shown at 496, the pair of lines for user 402 are shown at 497,
and the pair of lines for user 403 are shown at 498.
Data input to controller 495 for a particular user is coupled to
the lasers which transmit on the wavelength assigned to that user.
For example, data which is to be transmitted to user 401 is input
to controller 495 on the input line of lines 496. This data is
coupled to lasers 461 and 462 which transmit the data on wavelength
L.sub.1. The data transmitted by laser 462 arrives at user 401 on
segment 410 of fiber 406. The data transmitted by laser 461 arrives
at user 401 on segment 409 after having traversed segments 407 and
408.
Since each user is connected to central office 405 by two different
bi-directional communication paths, each user is protected against
any single break in fiber 406. Consider the case in which a break
occurs which prevents light from being transmitted on segment 408.
In particular, consider user 401. If the currently active detector
in the interface unit 420 at user 401 is the detector that receives
signals from laser 462 via segment 410, user 401 will not be
affected by the break. If, on the other hand, the detector in
question were the detector that received light from laser 461 via
segment 409, the controller 450 in the interface unit would detect
a loss of reception on segment 409. The controller 450 would then
make the other detector, i.e., the detector which receives light
from laser 462 via segment 410, the active detector.
It will be apparent to those skilled in the art that the controller
associated with each user will similarly switch detectors if
necessary to restore communication with central office 405. Hence,
this embodiment of the present invention provides communication
between several users and the central office on a single optical
fiber while preserving the "self-healing" aspects of the embodiment
shown in FIGS. 3 and 4. That is, the system automatically
reconfigures itself to compensate for a single break in the
fiber.
The number of users that can be accommodated on a single fiber is
determined by the sensitivity of the various detectors and the
amount of light generated by the various lasers. The light pipes in
each of the interface units 420 divert a portion of the available
light out of the fiber. If too many users are connected to a given
fiber, there will be too little light for the detectors in the
interface units 420 to operate. The number of users can increased
by increasing the amount of light generated by lasers 461-466.
Similarly, if beam splitters such as light pipes 490-491 are used
to illuminate detectors 471-476, the amount of light detected by
any given detector will be inversely related to the number of
users. It will be apparent to those skilled in the art that this
limitation may be removed by utilizing a device which spatially
separates different wavelengths to illuminate detectors 471-476.
For example, a prism or diffraction grating may be utilized. In
this case, all of the light at a given wavelength will be directed
to a single detector. Such an arrangement also eliminates the need
for wavelength filters in front of each of the detectors.
Alternatively, more powerful lasers could be included in each of
the interface units 420.
In the above described embodiments of the present invention, a
controller such as controller 450 selects one of the two
bi-directional communication paths to be the active bi-directional
communication path at any given time. It will be apparent to those
skilled in the art that the self-healing feature of the present
invention does not require that one communication path be so
defined provided a delay is introduced into the signals in one of
the detectors to compensate for different optical path lengths. In
this case, the controllers can be replaced by a simple OR circuit
which combines the outputs of the detectors.
Referring to FIG. 3, assume that fiber optic segment 76 is longer
than fiber optic segment 78. If a delay circuit is included in
detectors 88 and 102 which provides a delay equal to the difference
in signal transit time between segments 76 and 78, then controllers
104 and 115 may be replaced by OR circuits In this case, when both
segments are functional, identical signals will be ORed to produce
the output signal. If one segment is cut, the signal from the other
segment will determine the output of the OR circuit.
Unfortunately, this simplification requires the inclusion of a time
delay circuit in each detector. Further, the time delay in question
must be separately set for each user. Finally, if one of the
bidirectional communication paths fails because of a malfunction in
a detector, the output of the OR circuit may not represent the
signal on the correctly functioning communication path. For
example, suppose detector 87 fails such that it puts out a
continuous string of ones. Then the OR circuit will likewise
produce a continuous string of ones. Hence, active switching
between the two bi-directional communication paths is
preferred.
The above described embodiments of the present invention utilize
the same pair of wavelengths to communicate along each of the
optical fiber segments. That is, any given user uses one wavelength
to communicate with the central office and one wavelength to
receive signals from the central office. Referring to FIG. 3, the
wavelengths of lasers 83 and 84 are the same, and filters 99 and
100 are likewise the same. This arrangement is preferred because it
minimizes the number of different wavelengths at which light is
received and transmitted. However, it will be apparent to those
skilled in the art that systems in which four different wavelengths
are utilized to communicate information between each user and the
central office are also possible. For example, referring to FIG. 3,
user 72 could send information to the central office on segment 76
using a different wavelength from that used to send the same
information on segment 78. Similarly, central office 74 could use
different wavelengths to send information on segments 76 and 78 to
user 72. This embodiment of the present invention will be referred
to as the four wavelength embodiment below.
The descriptions of the embodiments of the present invention shown
in FIGS. 2-5 have assumed that all users are receiving redundant
service. If a given user does not require redundant service, that
user's interface to the optical fiber need include only one
transmitter and one receiver for transmitting and receiving light
on one of the two segments of the optical fiber which were created
by inserting the interface. If the optical distribution system
utilizes one fiber segment for each non-redundant service user,
then a second non-redundant service user can be connected to the
other optical fiber segment.
If the embodiment of the present invention utilizes one fiber for
servicing a plurality of users in the redundant service mode, i.e.,
the embodiment shown in FIGS. 4 and 5, a non-redundant service user
can be connected by utilizing an interface unit which includes only
one laser, one detector, and one "Y" shaped light pipe. For
example, such an interface unit would resemble interface unit 420
shown in FIG. 5 with light pipe 424, detector 442, and laser 446
removed. A second non-redundant user could be connected to the same
fiber by using an interface unit which resembles interface unit 420
shown in FIG. 5 with light pipe 422, detector 432, and laser 434
removed. By using pairs of "complementary" interfaces, the number
of non-redundant users that can be connected to a single fiber is
increased, since a pair of complementary interfaces removes the
same amount of light from the fiber as one interface 420 used for
providing redundant service. The four wavelength embodiment of the
present invention described above may be utilized when both
redundant and non-redundant service is provided on the same
fiber.
Although the above described embodiments of the present invention
have been described with reference to a telephone central office,
it will be apparent to those skilled in the art that the present
invention will also function in any system in which a plurality of
users must be connected to a "master" user in a manner which is
immune to cable breaks. Accordingly, there has been described
herein a fiber optic distribution system for use in telephone
systems and the like. Various modifications to the present
invention will become apparent to those skilled in the art from the
foregoing description and accompanying drawings. Accordingly, the
present invention is to be limited solely by the scope of the
following claims.
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